Pigment compositions for oil-based lithographic printing inks

ABSTRACT

Pigment compositions suitable for oil-based lithographic printing inks are provided which comprise besides an organic pigment, a hyperdispersant which is a poly-alkyleneiminepolyester adduct, a synergist agent which is a quaternary ammonium pigment derivative, a solvent which includes aliphatic/aromatic hydrocarbon distillates and vegetable oils, and optionally rosin or a modified rosin. The printing inks prepared from these pigment compositions show outstanding improvements in their rheological properties.

The present invention relates to pigment compositions suitable for usein oil-based lithographic printing inks. More particularly, theinvention relates to pigment compositions containing besides the pigmenta combination of additives comprising a synergist component and apolymeric hyperdispersant, dissolved in a solvent which is a hydrocarbondistillate fraction or a vegetable oil.

Lithographic printing is a process which utilizes a coated metal orpolymeric plate containing a hydrophobic image area which acceptshydrophobic based ink and a non-image hydrophilic area which acceptswater, i.e. the fount(ain) solution.

Many oil-based inks, especially vegetable oil-based lithographicprinting inks, are prone to an uptake of fount solution in areas ofshear, e.g. at the ink/fount contact where the ink duct rollers meet thefounted press drum. This intimate contact of the fount solution and theink causes an emulsification and the thus emulsified ink can “hang back”due to a high viscosity when emulsified. In extreme cases the ink willcease to flow onto the printing press. It is known that pigmentcompositions have a significant effect on this hang back phenomenon.

It has now been found that these problems can be overcome andoutstanding effects can be achieved when using the new organic pigmentcompositions hereinafter described which comprise a particularcombination of additives for the preparation of oil-based lithographicprinting inks.

Accordingly it is the main object of the present invention to providesaid new pigment compositions. Other objects of the present inventionrelate to processes to prepare said compositions, to prepare printinginks from said compositions and to use the inks in lithographic printingprocesses. These and other objects of the present invention will bedescribed in the following.

Therefore, in a first aspect of the present invention, there is provideda pigment composition comprising

-   (a) 60 to 90% of an organic pigment,-   (b) 1 to 10% of a hyperdispersant,-   (c) 1 to 10% of a synergist agent (additive),-   (d) 1 to 10% of a solvent, and-   (e) 0 to 40% of rosin or a modified rosin.

Preferred are e.g. the following percentage ranges: 60 to 80% ofcomponent (a), 2 to 6% of component (b), 2 to 6% of component (c), 3 to8% of component (d), and 2 to 30% of component (e).

All percentages are by weight.

The pigments of component (a) are those producing the four colourscommonly used in the printing industry: namely black, cyan (blue),magenta (red) and yellow. As a rule, they are compatible with the othercomponents of the inventive pigment compositions which constitute thebasis (colourant) for forming the oil-based printing inks forlithographic printing processes, which are another object of the presentinvention.

Organic pigments as component (a) comprise such as, but not exclusively,monoazo, disazo, azomethin, azocondensation, metal-complex azo,naphthol, metal complexes, such as phthalocyanines, dioxazone, nitro,perinone, quinoline, anthraquinone, hydroxyanthraquinone,aminoanthraquinone, benzimidazolone, isoindoline, isoindolinone,quinacridone, anthrapyrimidine, indanthrone, flavanthrone, pyranthrone,anthanthrone, isoviolanthrone, diketopyrrolopyrrole, carbazole,perylene, indigo or thioindigo pigments. Mixures of the pigments mayalso be used.

The disazo pigments of component (a) represent an important class ofcolouring materials (colourants) used commonly for the manufacture ofprinting inks. Preferably they are yellow and orange diarylide pigmentsand orange disazopyrazolone pigments, including e.g. the C.I. PigmentYellows 12, 13, 14, 17, 83, 174 and 188, as well as the C.I. PigmentOranges 13, 16 and 34 which are often used as shading agents. Furtherpreferred are metal complexes, such as copper phthalocyanine pigments(e.g. C.I. Pigment Blue 15:3), or naphthol pigments, preferablyβ-naphthol or β-oxynaphthoic acid (BONA) pigments (e.g. C.I. Pigment Red57:1).

For further details as to all these organic pigments reference is madeto Industrial Organic Pigments, W. Herbst, K. Hunger, 2^(nd) edition,VCH Verlagsgesellschaft, Weinheim, 1997. The so-called hyperdispersantsof component (b) are e.g. reaction products of a poly(loweralkylene)-imine with a polyester having a free carboxylic acid group, inwhich there are at least two polyester chains attached to eachpoly(lower alkylene)-imine.

The reaction product may be a salt or an amide depending on the severityof the reaction conditions under which the polyester is reacted with thepoly(lower alkylene)-imine.

A preferred polyester is derived from a hydroxycarboxylic acid of theformula OH—X—COOH, wherein X is a divalent saturated or unsaturatedaliphatic radical containing at least 8 carbon atoms, preferably 12 to20 carbon atoms, and in which there are at least 4, preferably 8 to 14carbon atoms between the carboxylic and the hydroxy groups.

As specific examples of such hydroxycarboxylic acids there may bementioned ricinoleic acid, a mixture of 9- and 1-hydroxystearic acids,and 12-hydroxystearic acid, and especially the commercially availablehydrogenated castor oil fatty acid which contains in addition to12-hydroxystearic acid minor amounts of stearic acid and palmitic acid.

The polyester can for example be obtained by heating thehydroxycarboxylic acid or a mixture thereof, optionally in the presenceof an esterification catalyst, at a temperature in the region of about160 to 200° C.

The term lower alkylene refers to alkylene groups containg 2 to 4 carbonatoms and the preferred poly(lower alkylene)-imine is polyethylene iminewhose molecular weight range is generally from 500 to 100'000,preferably from 10'000 to 100'000.

Further details and examples of component (b) are disclosed in GB2'001'083, the substance of which is Incorporated herein by reference.

The so-called synergistic additive (agent) of component (c) is forexample an asymmetric disazo compound comprising a central divalentgroup, free from ionic substituents, linked through azo groups to twomonovalent end groups, the first being free from any ionic groups andthe second being a single substituted ammonium salt group.

The central divalent group of the asymmetric compound is preferably abiphenylene group which may be unsubstituted or substitued by one ormore non-ionic groups selected from lower alkyl, lower alkoxy (lowermeans C₁₋₄), halogen (chloro), nitro and cyano.

The first end group of the asymmetric compound, which is free from ionicsubstituents, is preferably a pyrazolin-5-on-4-yl, a2-hydroxynaphth-1-yl or an acetoacet-2-ylanilide group, such groupsbeing typically present in disazo pigments. They may carry substituentssuch as lower alkyl, lower alkoxy, halogen, nitro, cyano, loweralkoxy-carbonyl, phenylaminocarbonyl, naphthylaminocarbonyl and phenyl,in which the phenyl and naphthyl groups are optionally substituted bylower alkyl, lower alkoxy, halogen, nitro or cyano.

The second end group of the asymmetric compound, carrying the saltgroup, may be otherwise identical to the first end group or may beselected from the first end groups defined above with the addition ofthe salt group. The second end group is preferably anacetoacet-1-ylanilide group in which the salt group is in the 4-positionon the benzene ring with respect to the amino group, a1-phenylpyrazolin-5-on-4-yl group in which the salt group is in the4-position on phenyl, or a 2-naphth-1-yl group in which the salt groupis in the 6-position of the naphthalene ring.

The substituted ammonium-acid salt group is preferably a substitutedammonium carboxylate or phosphonate group or especially a substitutedammonium sulfonate group. The substituted ammonium-acid salt grouppreferably contains at least one fatty aliphatic group attached to thenitrogen atom of the ammonium ion. The substituted ammonium ioncontains—as a rule—at least 6, preferably at least 12, and morepreferably from 16 to 80, carbon atoms in from 1 to 4 aliphatic groups.In a particular useful agent the ammonium ion has 3 or 4 aliphaticgroups containing in total from 16 to 60 and more preferbly from 25 to40 carbon atoms. It is also preferred that at least one of the aliphaticgroups contains 8 to 20, especially preferred 26 to 20 carbon atoms.

Examples of the substituted ammonium compounds, e.g. halides andhydroxides, which may be used to prepare component (c) are tallow benzyldimethyl ammonium chloride, ditallow dimethyl ammonium chloride,ditallow benzyl methyl ammonium chloride, coco benzyl dimethyl ammoniumchloride and dicoco dimethyl ammonium chloride.

Further details and examples of component (c) are disclosed in EP 0 076024, the substance of which is incorporated herein by reference.

Component (d) constitutes a solvent (ink vehicle) which may be aso-called mineral oil solvent which comprises aliphatic or aromatichydrocarbon distillate fractions of boiling points of from 100 to 350°C., preferably of from 180 to 300° C., or vegetable oils.

The vegetable oils for use in the printing ink vehicles of the inventionare the commonly available vegetable triglycerides in which the fattyacid moieties have a chain length of about 12 to 24 carbon atoms,preferably of 18 to 22 carbon atoms. Of particular interest are thosewhich have a substantial proportion of diunsaturated linoleic fatty addand triunsaturated linolenic fatty acid moieties, e.g. soybean, coconut,cottonseed, linseed, safflower, sunflower, corn, sesame, rapeseed andpeanut oil or mixtures thereof.

Though the aforementioned oils can be employed in the crude state asoriginally expressed from the seed material, there are advantages tosubjecting them to certain preliminary processing steps. For example,alkali refining removes the gums and phospholipids which may interferewith the properties of the vehicles and the ultimate ink formulations.Alkali refining also removes free fatty acids, which tend to reducehydrophobicity properties in ink formulations.

The hydrocarbon distillate fractions as component (d) are preferred, butvegetable oils are also important.

Component (d) may be added separately to the inventive pigmentcompositions, but preferably it may be added together with component(b), i.e. as a solution of component (b) in component (d).

The optional component (e) includes—but is not limited to—rosin (abieticacid), rosin (acid) salts, such as alkali metal salts (sodium,potassium), and modified rosins such as rosin (acid) metal resinates(copper, zinc, magnesium resinates), rosin esters, such as maleinizedrosin, pentaerythritol rosin or rosin-modified phenolic resins, andfurther vegetable oil based rosin esters, such as soybean or tall oilesters (methyl, butyl), and further hydrogenated rosins,disproportionated rosins, dimerised, polymerised and part-polymerisedrosins (rosins, cross-linked with e.g. formalsehyde), or mixturesthereof. These compounds and their use in printing compositions are wellknown in the art.

The pigments of the inventive pigment compositions may be prepared byfollowing processes including various conventional steps well known inthe art; components (b), (c), (d) and optionally (e) may be added duringthese steps to prepare the inventive pigment compositions.

Alternative methods that may be used are e.g.:

-   (I) straight addition of the components during any of the    conventional steps;-   (II) emulsification with water using a suitable surfactant;-   (III) emulsification with a micellar resin soap solution;-   (IV) separate addition of a water-soluble carrier solution of    component (b), followed by a separate addition of the other    components (in any of steps (I) to (III) inclusive)

The synergist additive (c) can also be added as a dry blend to thepigment powder during the milling step in the pigment preparation.

The inventive pigment composition may be used to prepare oil-basedprinting inks for lithographic printing processes. As a rule, such anink contains about 5 to 50% by weight of the pigment composition.

Furthermore, the lithographic printing inks may in addition comprisecustomary additives known to those skilled in the art.

Typical additives include drying enhancers, drying inhibitors,non-coloured extenders, fillers, opacifiers, antioxidants, waxes, oils,surfactants, rheology modifiers, wetting agents, dispersion stabilizers,strike-through inhibitors and anti-foaming agents; further adherencepromoters, cross-linking agents, plasticisers, photinitiators,deodorants, biocides, laking agents and chelating agents.

Such additives are usually used in amounts of from 0 to 5% by weight,particularly from 0 to 2% by weight, and preferably from 0.01 to 1% byweight, based on the total weight of the lithographic printing inkcomposition.

The inks and the printing processes are further objects of the presentinvention.

The inventive pigment composition is dispersed into the lithographicprinting ink system, which is preferably a vegetable oil system, byconventional means, e.g. by premixing, then beadmilling using either ahorizontal or vertical beadmill or by premixing of the pigment into thevarnish followed by dispersion on a three-roll mill.

The millbases is usually let-down with more varnish components and waxadditives to adjust the final ink properties, such as a distinctrheological behaviour (flow) and tackiness.

The inventive printing ink can be used on a lithographic printing presswhereby it is passed from a reservoir by means of a roller duct systemto the inking plate. This plate is pre-treated with aqueous fountsolution often containing alcoholic components to aid the lithographicprocess. At the interface of the ink duct roller and inking roller thefount solution becomes intimately contacted with the ink causing anemulsification. In extreme cases the ink will cease to flow and “hangback” because the water increases the complex viscosity of the ink toomuch. The inventive printing inks overcome this drawback by reducing thecomplex viscosity of the ink when the fount (solution) is emulsified inthe ink and thus the ink continues to flow onto the press in anappropriate and effective manner.

The inventive printing inks produce the desired Theological propertiesin all types of lithographic printing inks know in the art, e.g.heatset, sheetfed or coldset printing inks based on aromatic andpreferably aliphatic hydrocarbon distillates or vegetable oils. Thevegetable oils, such as preferably linseed or soybean oil, but alsoalkylesters (methyl, butyl) of tall oil rosins, are preferred over thedistillates. These systems are more polar in nature and are thereforemore susceptible to the uptake of water in emulsified form, particularlywhen there is an alcoholic component present in the fount solution.

It is the particular combination of components (b), (c) and (d) (andoptionally (e)) of the inventive pigment composition which isresponsible for and achieves the advantageous effects of the inventivelithographic printing inks with regard to their Theological properties(good wet and dry flow). Optionally, the wet flow advantage can beachieved already by the combination of components (b) and (d), and thedry flow advantage by component (c), alone.

The present invention is hereinafter further described with reference toparticular examples thereof. It will be appreciated that these examplesare presented for illustrative purposes and should not be construed as alimitation of the scope of the invention as herein described.

In the following examples, quantities are expressed as part by weight orpercent by weight, if not otherwise indicated. The temperatures areindicated in degrees centigrade.

EXAMPLES Example 1

General manufacturing instruction (1): A diarylide yellow pigment (C.I.Pigment Yellow 13, C.I. No. 21100) is prepared by coupling anaceto-acetyl compound (aceto-acet-2,4-xylidene) by forming a basicsolution thereof followed by re-precipitation of the free acid form ofthe aceto-acet compound by the addition of a mixture of acetic andhydrochloric acid. This ‘seeded’ coupling component has a solution oftetrazotised 3,3′-dichlorobenzidine added over about 1 hour at 15 to 20°C. and a pH-value of 4.5 to 6.0.

The tetrazotised 3,3′-dichlorobenzidine is prepared by the addition ofexcess hydrochloric acid and sodium nitrite solution to an aqueousslurry of 3,3′-dichlorobenzidine dihydrochloride at 0 to 10° C.

The resultant amorphous pigment is then treated with a rosin (acid)sodium salt and a 40% solution of the copolymer of poly-(12-hydroxystearic acid) (hyperdispersant, component (b)) in an (aromatic free)distillate of a boiling point range of 240 to 260° C.

The resultant slurry is heated to 90-93° C. by the addition of directsteam, and then the pH is slowly adjusted to 5. The slurry is flushedback to 70° C. and then an aqueous slurry of a synergist additive(quaternary ammonium pigment derivative, component (c)) is added andstirred out.

The slurry is then filtered, washed and dried until the moisture andresidual salt contents are both less than 1% by weight, respectively.The pigment retains the added components quantitatively after saidwashing and drying steps.

Component (b):

Copolymer of polyethylene imine (molecular weight of about 50'000) andpoly-(12-hydroxy stearic acid (obtained by heating 12-hydroxystearicacid for about 20 hours a 190-200° C.)

Component (c):

Coupling of an equimolar mixture of acetoacetanilide andacetoacetanilide-4-sulphonic acid (potassium salt) with tetrazotised3,3′-dichlorobenzidine. The resultant (yellow) compound is ion-pairedwith dihydrogenated tallowdimethyl ammonium chloride.

According to the general manufacturing instruction the following pigmentcomposition is prepared.

According to the Present Invention: C.I.Pigment Yellow 13 (component(a)) 65.0% Copolymer (component (b))  3.0%¹⁾ Synergist additive(component (c))  3.0% Aromatic-free distillate (component (d))  4.0%¹⁾(boiling point: 240-260° C.) Rosin (acid) (component (e)) 25.0%¹⁾combined 40:60

Comparative Example 1A

C.I. Pigment Yellow 13 (component (a)) 70.0% Rosin (acid) (component(e)) 30.0%

This pigment composition is then dispersed into a lithographic printingink system (percentage of the composition present in the ink system: 5to 50%) by conventional means (milling). The ink shows excellentrheological properties, especially in regard to duct flow of the dry inkand hang back of the wet ink.

The inks are tested for their low shear flow properties as correlationwith their flow properties on a lithographic printing press. The lowshear flow performance correlating with the ink's duct flow and hangbackperformance when considered as dry ink in the first case and wet oremulsified ink in the second case (“inclined plate test”). Duct Flow ofDry Ink: Example 1 (invention): 6.6 cm Example 1A (comparison): 3.2 cmHang Back of the Wet Ink: Example 1 (invention): 4.5 cm Example 1A(comparison): 2.1 cm

Example 2

General Manufacturing Instruction (2):

A copper phthalocyanine pigment (C.I. Pigment Blue 15:3, C.I. No. 74160,Component (a)) is prepared by reaction of phthalic anhydride, urea and acopper source such as CuCl₂ in the presence of an aromatic solvent (i.e.o-nitrotoluene) and a molybdate catalyst under increased temperature andpressure. The resultant so-called crude copper phthalocyanine (i.e.having crystals of −200 μm and >90% β-phase) is then subjected to drymilling (i.e. ball milling) in the presence of an inorganic salt such asNaCl along with wood rosin (Component (e)). The milled intermediate,containing ˜50% β-phase, is then solvent conditioned in a basic (pH>11)solution containing an organic solvent such as n-butanol at elevatedtemperature until a pigmentary (i.e. <10 μm) copper phthalocyaninecomposition containing >90% β-phase is obtained. The pigment is thenfiltered and washed solvent and salt free (<300 μS conductivity) andretained as a 46.4% solids aqueous press cake.

The press cake is then re-dispersed in water an treated with a rosinacid salt (Component (e′)) and a 40% solution of the co-polymer ofpoly-(12-hydroxystearic acid) (hyperdispersant, Component (b)) in an(aromatic free) petroleum distillate (Component (d)). The resultantslurry is heated to 90° C. using e.g. a water bath and stirredmechanically for 60 minutes before being treated with a quaternaryammonium pigment derivative (synergist, Component (c)) and cooledimmediately using no artificial means. Once below 35° C., the slurry isacidified (pH<1) using concentrated HCl and finally stirred out.

The pigment slurry is then filtered, washed acid and salt free (<300 μSconductivity) and dried at 70-80° C. The pigment retains components(a)-(e′) after the washing/drying steps.

Modified General General Manufacturing Instruction (2.1):

A copper phthalocyanine pigment (C.I. Pigment Blue 15:3, Component (a))is prepared by dry-milling crude Copper Phthalocyanine in the presenceof an inorganic salt such as NaCl and wood rosin (Component (e)),followed by a solvent conditioning stage which is carried out in anaqueous solution containing an organic solvent. The resulting presscakefrom this preparation is then re-dispersed in water and treated with arosin acid salt (Component (e′)) and a 40% solution of the co-polymer ofpoly-(12-hydroxystearic acid) (hyperdispersant, Component (b)) in an(aromatic free) petroleum distillate (Component (d)). The resultantslurry is heated to 90° C. using e.g. a water bath and stirredmechanically for 60 minutes before being treated with a quaternaryammonium pigment derivative (synergist, Component (c)) and cooledimmediately using no artificial means. Once below 35° C., the slurry isacidified (pH<1) using concentrated HCl and finally stirred out.

The pigment slurry is then filtered, washed acid and salt free (<300 μSconductivity) and dried at 70-80° C. The pigment retains components(a)-(e′) after the washing/drying steps.

Component (a):

-   Pigmentary copper phthalocyanine, C.I. Pigment Blue 15:3    Component (e):-   Partially hydrogenated wood rosin (Staybelite®)    Component (e′):-   Disproportionated wood rosin, potassium salt (Burez®)    Component (b):-   Copolymer of polyethylene imine (MW ˜50,000) and    poly-(12-hydroxystearic acid) (Solsperse® 13,000)    Component (d):-   Aromatic free petroleum distillate having a boiling range of    240-270° C.    Component (c):-   Dihydrogenated tallow dimethyl ammonium chloride ion-paired with    copper phthalocyanine mono-sulphonic acid (Solsperse® 5000)

According to the general manufacturing instructions outlined above, thefollowing pigment preparations are prepared.

According to the Present Invention: C.I. Pigment 15:3 (component (a))80.0% Rosin 1 (component (e)) 2.3% Rosin 2 (component (e′)) 3.3%Hyperdispersant (component (b)) 4.1% Distillate (component (d)) 6.2%Synergist (component (c)) 4.1%

Comparative Example 2A

C.I. Pigment 15:3 (component (a)) 80.0% Rosin 1 (component (e))  2.3%Rosin 2 (component (e′)) 17.7%

The pigment compositions are then dispersed into a lithographic inksystem (percentage of the compositions in the ink system: 5-50%) byconventional means (triple roll milling). The ink obtained in example 2shows excellent rheological properties, especially in regard to ductflow of the dry ink and hang back of the wet ink compared withcomparative example 2A.

Duct Flow of Dry Ink: Example 2 (invention): 10.5 cm Example 2A(comparison):  7.5 cm

Hang Back of the Wet Ink: Example 2 (invention): 14.2 cm Example 2A(comparison):  6.5 cm

Example 3

General manufacturing instruction (3): A Ca4B pigment (C.I. Pigment Red57:1, C.I. No. 15850) is prepared by coupling beta-hydroxynaphthoic acidby forming a basic solution thereof followed by addition of a rosin(acid) sodium salt and a 40% solution of the copolymer of polyethyleneimine (MW ˜50,000 and poly-(12-hydroxystearic acid) (hyperdispersant,component (b)) in an (aromatic free) distillate of a boiling point rangeof 240 to 260 degrees C. This mixture and a slurry of diazotised4-aminotoluene-3-sulphonic acid are added together into the couplingvessel over about 23 minutes at 8 to 10 degrees C. and a pH value of10.8 to 11.0. The diazotised 4-aminotoluene-3-sulphonic acid is preparedby the addition of excess hydrochloric acid and sodium nitrite solutionto basic aqueous solution of 4-aminotoluene-3-sulphonic acid at 0 to 10degrees C.

The pH of the resultant slurry is slowly adjusted to 7.2 and then heatedto 90 degrees C. by the addition of direct steam. After being held atthis temperature for 15 minutes, the slurry is flushed back to 70degrees C. and then a synergist additive (quaternary ammonium pigmentderivative, component (c)) is added and stirred out.

The slurry is then filtered, washed and dried until the moisture andresidual salt contents are both less than 1% by weight, respectively.The pigment retains the added components quantitatively after saidwashing and drying steps.

Component (b):

-   as described in Example 1.

Component (c):

Coupling of an equimolar mixture of beta-naphthol andbeta-naphthol-6-sulphonic acid (potassium salt) with tetrazotised3,3′-dichlorobenzidine. The resultant (red) compound is ion-paired withdihydrogenated tallowdimethyl ammonium chloride.

The following pigment compositions are prepared:

According to the Present Invention: C.I.Pigment Red 57:1 (component (a))67.1% Copolymer (component (b))  5.3% Synergist additive (component (c)) 3.9% Aromatic-free distillate (component (d))  7.9% Rosin (acid)(component (e)) 15.8%

Comparative Example 3A

C.I. Pigment Red 57:1 (component (a)) 70.0% Rosin (acid) (component (e))30.0%

This pigment compositions are then dispersed into a lithographicprinting ink system (percentage of the composition present In the inksystem: 5 to 50%) by conventional means (milling). The ink obtained inexample 3 shows excellent rheological properties, especially in regardto duct flow of the dry ink and hang back of the wet ink when comparedto comparative example 3A. Duct Flow of Dry Ink: Example 3 (invention):18 cm Example 3A (comparison): 13 cm Hang Back of the Wet Ink: Example 3(invention): 14 cm Example 3A (comparison): 11.5 cm

1. A pigment composition comprising (a) 60 to 90% of an organic pigment,(b) 1 to 10% of a hyperdispersant, (c) 1 to 10% of a synergist additive,(d) 1 to 10% of a solvent, and (e) 0 to 40% of rosin or a modifiedrosin.
 2. The pigment composition according to claim 1, wherein theorganic pigment (a) is a disazo pigment, a metal complex pigment, or anaphthol pigment.
 3. The pigment composition according to claim 1,wherein the hyperdispersant (b) is a reaction product of a poly(loweralkylene)-imine with a polyester having a free carboxylic acid group, inwhich there are at least two polyester chains attached to eachpoly(lower alkylene)-imine.
 4. The pigment composition according toclaim 3, wherein the hyperdispersant (b) is a reaction product ofpoyethyleneimine of a molecular weight range of 500 to 100'000 with apolyester derived from a hydroxycarboxylic acid of the formulaHO—X—COOH, wherein X is a divalent saturated or unsaturated aliphaticradical contaning at least 8 carbon atoms, and in which there are atleast 4 carbon atoms between the carboxylic and the hydroxy groups. 5.The pigment composition according to claim 1, wherein the synergisticadditive (c) is an asymmetric disazo compound comprising a centraldivalent group, free from ionic substituents, linked through azo groupsto two monovalent end groups, the first being free from any ionic groupsand the second being a single substituted ammonium salt group.
 6. Thepigment composition according to claim 1, wherein the solvent (d) is analiphatic or aromatic hydrocarbon distillate fraction of boiling pointsof the range of 100 to 350° C. or a vegetable oil.
 7. The pigmentcomposition according to claim 6, wherein the vegetable oil is atriglyceride in which the fatty acid moieties have a chain length of 12to 24 carbon atoms.
 8. The pigment composition according to claim 1,wherein the modified rosin (e) is a rosin (acid) metal resinate, a rosinester, a pentaerythritol rosin and a rosin-modified phenolic resin, avegetable oil based rosin ester, a hydrogenated rosin, adisproportionated rosin, or a dimerised, polymerised or part-polymerisedrosin, or mixtures thereof.
 9. An oil-based printing ink forlithographic printing containing as colourant a pigment compositionacording to claim
 1. 10. The printing ink according to claim 9containing as colourant 5 to 50% of the pigment composition, andoptionally further customary additives.
 11. A process for preparing theprinting ink according to claim 9 which comprises dispersing the pigmentcomposition into a lithographic printing ink system.
 12. A process forpreparing the printing ink according to claim 10 which comprisesdispersing the pigment composition into a lithographic printing inksystem.
 13. The pigment composition according to claim 2, wherein thedisazo pigment is a diarylide pigment.